This invention relates to device capable of interacting with and modifying at least one characteristic of incident microwave radiation. More particularly this invention is directed to an optically pumped multi-component chemical system capable of producing high electron spin polarization at room temperature and the use of such system in maser construction.
This invention relates to the use of a special class of chemical systems, which, upon light excitation in an external magnetic field, significantly change their magnetic permeability in the microwave region. The invention makes possible the construction of new classes of microwave devices, including ultra low noise microwave amplifiers, phase shifters with very low losses and electromagnetic devices for the protection of sensitive receivers from strong microwave pulses.
The generation of high controllable magnetic permeability in paramagnetic materials was long ago recognized as being important for variety of microwave devices (Wittke). The original treatment of Wittke dealt with the change in the electron spin population of the Zeeman levels as a mean to perturb the electromagnetic (EM) radiation in the matter, mainly to amplify it. The population difference in the Zeeman levels (xcex94N) can be related directly to the material""s magnetic permeability, which is more naturally used in relation to various EM applications by the following expressions:
The magnetic permeability is defined in cgs units by:
xcexc=1=4xcfx80xcexaxe2x80x83xe2x80x83(1) 
where xcexa="khgr"/xcfx81 is the volume magnetic susceptibility "khgr" is the mass magnetic susceptibility per gram, and xcfx81 is the material density. It is also known that for a magnetic transition in 2S+1 energy levels of a spin system (McMillian):
P(mS)=xcex94Nxc2x7hxcexdxc2x7xcfx81(mS)=xcex94Nxc2x7hxcexdxc2x7(xcfx80/4)xcex32H12(S+mS)(Sxe2x88x92mS+1)f(xcexdxe2x88x92xcexd0) xe2x80x83xe2x80x83(2) 
where P(ms) is the power absorbed by spin system (mSxe2x88x921 to mS), xcexd is the microwave frequency, xcex3 is the electron gyromagnetic ratio, H1 is the magnetic part of the microwave field, f(vxe2x88x92v0) is the normalized absorption/emission line shape function, which depends on the frequency v and attains its maximum at xcexd0 and p(mS) is the probability of transition per time unit. The power at frequency xcexd, absorbed by a magnetic system with an imaginary part of the volume magnetic susceptibility, xcexaxe2x80x3, which is the physical parameter important for practical applications is:
P(ms)=xcfx80xcexdxcexaxe2x80x3H12 xe2x80x83xe2x80x83(3) 
This expression can be either positive or negative, depending on the sign of xcexaxe2x80x3, implying the ability to absorb or amplify microwave radiation. Thus, in terms of eq. 2, xcexaxe2x80x3 is expressed as a function of xcex94N between the magnetic levels:                               κ          xe2x80x3                =                                                            Δ                ⁢                                  xe2x80x83                                ⁢                N                            4                        ·            h                    ⁢                      xe2x80x83                    ⁢                                                    γ                2                            ⁡                              (                                  S                  +                                      m                    s                                                  )                                      ·                          (                              S                -                                  m                  s                                +                1                            )                                ⁢                      f            ⁡                          (                              v                -                                  v                  0                                            )                                                          (        4        )            
The permeability (xcexc) of common paramagnetic systems is very close to unity, implying that xcexaxe2x80x3xcx9c0, with a negligible effect due to microwave excitation. For applied and practical purposes, xcexaxe2x80x3 must be of the order of 0.001-0.01 in the microwave frequencies relevant for the present applications as determined by the external magnetic field. These values ensure that for applications such as phase shifters or microwave amplifiers, the change in the microwave power, i.e., absorption or emission (cf eq. 2) due to interaction with the paramagnetic material is large enough. For example, in the case of the microwave amplifier, the amplification due to xcexaxe2x80x3, must be much larger than the dielectric losses in the material which always exist. The xe2x80x9cthreshold valuesxe2x80x9d for xcexaxe2x80x3 listed above, depend upon the specific microwave structure in which the material is inserted and the material dielectric properties. The values of xcexaxe2x80x3 (0.01-0.001) are typical representative figures known for microwave amplifiers (Yariv) which are similar to the measured and calculated values of the present systems (see below).
Since paramagnetic materials at room temperature have relatively very low magnetic permeability (xcex94N is very small), they can not be exploited for practical purposes. Thus one must either go to very low temperatures, or use some pumping mechanism to increase the population difference in the magnetic levels, and by that, to increase the magnetic permeability. The solid state microwave masers, which were common in the 1950""s and 1960""s used both cooling and microwave pumping to achieve relatively high permeability (Orton et al). These masers can operate only at very low temperatures, typically in the order of 2 K (xe2x88x92271 xc2x0 C.) or even less, a restriction that precluded a widespread use of these devices as amplifiers. In the 1970""s, a new type of solid state amplifier based on the field effect transistor (FET), followed in the 1980""s by the high electron mobility transistor (HEMT) appeared. Amplifiers based on these transistors can operate at cryogenic temperatures with similar noise performance achieved by solid state maser amplifiers, at least up to frequencies of several GHz. With these transistors, and the improved technology of solid state electronics, masers were gradually removed from the scene. However, it is important to note that even today, masers are used in some specific applications where noise performance is crucial, such as radio astronomy (Glass) and in astronomic radars (for example, the Arecibo Planetary Radar for radio astronomy).
The restriction of low temperature operation in the solid state masers of the 1960""s was mainly due to two reasons. First, the pumping mechanism was in the microwave region. Thus, in order to create high population inversion, kbT (Boltzmann constant multiplied by the temperature) must be as small as possible, compared to hxcexd (Planck constant multiplied by the frequency of radiation). The second reason was that the solid state materials, which were used as the active material in the masers, have very steep dependence of the spin relaxation time of the magnetic levels upon temperature. Thus at higher temperatures, the fast relaxation can not permit efficient pumping of the magnetic levels, and the population of the levels does not change much. Using optical excitation to pump the magnetic levels can solve the first problem. These are called solid state optically pumped masers (Anderson et al U.S. Pat. No. 3,736,518). With optical excitation one can, in principle, operate the maser at higher temperatures, as the pumping is done with a much larger hxcexd. However, again, the second constrain of short relaxation time of the magnetic levels begins to be of importance and the improvement in the temperature of operation is very small (operation in about 10 K instead of 2 K).
In parallel to these efforts, in the 1960""s and 1970""s, there were some new discoveries which showed how one can produce high population inversion in paramagnetic materials by chemically induced process, named CIDEP (chemically induced dynamic electron polarization (Muus et al)). These processes were related to the production of paramagnetic species in chemical reactions with non-Boltzmann spin populations, known as electron spin polarization. Although the initial discoveries were made in relation to non-reversible chemical reactions, later studies demonstrated this phenomenon in photophysical reversible light induced processes. In the late 1980""s, a new observation of electron spin polarization generated through the interaction of photoexcited triplets and stable radicals in solution. This reversible physical process is called RTPM (radical triplet pair mechanism) and results in very high population inversion in the stable radical (Blxc3xa4ttler et al). The reversibility of the process and the ability to control the radical polarization by means of solution viscosity and type of triplet used, makes this mechanism very attractive in terms of practical application. This is the mechanism on which we base our inventions. The RTPM, in contrast to all previous attempts to generate optically pumped masers, including our previous efforts (Blank et al), is a reversible, photoinduced, intermolecular process. This gives it a substantial lead over all previous intramolecular (such as optically pumping, and, intramolecular intersystem crossing of the C60 molecule (Blank et al.)). By employing intermolecular process, one can have very strong control over the dynamics (magnitude of magnetization) and kinetics (the magnetization time dependency) by changing one or two of the pair of molecules involved (radical and triplet) and by changing the solvent properties, such as viscosity. Such control is not possible in intramolecular processes, where to change things, one must change the quantum mechanical properties of the molecular systems, a task which is difficult and out of our current interest. Currently, there are several theoretical treatments, which can provide fairly good prediction of the radical""s polarization and the total magnetization as function of the radical and triplet concentrations, as well as the solvent properties. These calculations involve the solution of the appropriate quantum mechanical equations and calculations of the reaction kinetics (described in Shushin""s and in Blank""s and Levanon""s papers in the professional journals). These theoretical predictions enable us to determine which radical and chromophores should be chosen and what solvents should be used to optimize the process of magnetization. The chemical systems chosen for this project will be described below.
In one embodiment of the invention there is provided an apparatus for modifying a microwave signal either by amplifying it, by shifting its phase or by limiting it above certain power levels. The apparatus comprises a dielectric shielded cavity having at least one light aperture and a wave guide element for directing microwave signals to and from the cavity. The apparatus further comprises a spin polarization matrix in the shielded cavity. The matrix itself comprises a stable free radical species and a light-induced chromophore, which are in communication through energy transfer. The stable free radical species can be in a crystalline matrix comprising the chromophore, or the stable free radical species and the chromophore can be combined, preferably in substantially equimolar amounts, in an optically transparent fluid medium which itself can comprise an organic solvent, a solvent miscible oil or a liquid crystal polymer composition. The spin polarization matrix is typically purged of oxygen and sealed in a dielectric optically transparent container, for example, a quartz vessel which is then located in shielded cavity.
The apparatus further comprises two additional elements critical for device function: a magnet for applying a substantially homogeneous magnetic field to the spin polarization matrix in the shielded cavity and a light source for radiating the matrix through the light aperture in the cavity to generate paramagnetic triplets from the chromophore component of the matrix. The magnet can either be a permanent magnet or an electromagnet or a combination of permanent and electro magnets, and can optionally include a probe for monitoring the magnetic field in the cavity.
The light source is preferably a source of visible light, and in one preferred embodiment it is a laser emitting light at a wave length optimally absorbed by the chromophore for efficient paramagnetic triplet formation. The light source is preferably a pulsed laser although the system can be adapted to continuous wave operation. Pulsed laser sources should have a power capacity such that the number of photons in each pulse is at least equivalent to the number of chromophore species in the matrix.